When multi-process windows in Firefox is enabled then Firefox will use more memory and may be less responsive. Try to disable multi-process windows in Firefox to see if that has effect.

You can disable multi-process windows in Firefox by setting these prefs to false on the about:config page.

• browser.tabs.remote.autostart = false
• browser.tabs.remote.autostart.2 = false

You can open the about:config page: via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

• http://www.ghacks.net/2016/07/22/multi-process-firefox/

Have you been reading Facebook, Twitter, or another page that has "endless scrolling" with embedded videos? Firefox seems not to discard any of the content on such pages so they can gobble up a lot of memory. One of the advantages of the 64-bit version of Firefox is that you can use much more memory; 32-bit Firefox grinds to a halt over about 2GB.